1. Field of the Invention
The present invention relates to a method of forming a curled portion on an end of a hollow body such as an automotive wheel rim or the like.
2. Description of the Related Art
There have widely been used in the art two-piece wheels for mounting automotive tires or the like. Such a two-piece wheel comprises a wheel rim (hereinafter simply referred to as “rim”) having a substantially hollow cylindrical shape formed from a sheet material and a wheel disc (hereinafter simply referred to as “disc”) having a substantially circular shape.
The rim is manufactured as follows. First, the ends of a rectangular sheet are brought into abutment against each other, producing a hollow cylindrical body. Then, the abutting ends are joined to each other by resistance welding, friction stir welding, or the like. The hollow cylindrical body is then held by a die having a certain recess. Thereafter, a spinning process is performed on the circumferential wall of the hollow cylindrical body, producing a rim having a recess called a drop defined in the circumferential wall thereof which extends in the circumferential direction.
Then, as disclosed in Japanese Laid-Open Patent Publication No. 2005-052884, the edges of the rim are curved into curled portions which make the rim stronger.
The curled portion is produced by a process shown in FIGS. 7A through 7E of the accompanying drawings. First, as shown in FIG. 7A, an edge of a rim 1 is preliminarily bent by a curling roller, not shown. The edge of the rim 1 is now curved toward the other edge thereof, producing a curled portion 2. At this time, the curled portion 2 has a substantially circular cross-sectional shape.
Then, the curled portion 2 is machined into a substantially rectangular cross-sectional shape (see FIG. 7E). An upper portion of the curled portion 2 in FIG. 7A is pressed flatwise as shown in FIG. 7B by a flattening roller, not shown.
As shown in FIG. 7C, a terminal end 3 of the curved portion 2 is compressed toward a circumferential wall 4 of the rim 1 by the curling roller.
As shown in FIG. 7D, an intermediate portion 5 of the curled portion 2 which extends substantially parallel to the circumferential wall 4 is pressed substantially flatwise by the curling roller, bringing the terminal end 3 closer to the circumferential wall 4.
Finally, a portion of the curled portion 2 near the terminal end 3 is pressed substantially flatwise by the curling roller. As shown in FIG. 7E, the curled portion 2 thus machined includes the intermediate portion 5 which extends substantially parallel to the circumferential wall 4, and a first perpendicular portion 6 and a second perpendicular portion 7 which extend substantially perpendicularly to the intermediate portion 5. The curled portion 2 is hollow and has a substantially rectangular cross-sectional shape.
The curled portion 2 is of high mechanical strength as it includes the first perpendicular portion 6 and the second perpendicular portion 7. The curled portion 2 is also excellent in appearance.
A hump, not shown, is then formed on the circumferential wall 4 of the rim 1. Thereafter, a disc having a hub hole, bolt holes, and decorative holes for better design and heat radiation is inserted into the rim 1. The rim 1 and the disc are joined to each other by MIG welding or the like, producing a wheel.
Occasionally, the curled portion 2 of the rim 1 has buckling creases 8 as shown in FIG. 8 of the accompanying drawings. The buckling creases 8 are formed because when the edge of the rim 1 is curved radially outwardly, the edge is increased in diameter, and when the curved edge, i.e., the curled portion 2, is machined into the substantially rectangular cross-sectional shape, compressive forces are applied along the circumferential directions of the curled portion 2. In other words, the buckling creases 8 are produced if the material of the curled portion 2 which is increased in diameter is not sufficiently compressed.
One solution would be to provide cuts in the edge of the rim 1 before it is curved in order that no buckling creases will be formed on the curled portion, as disclosed in Japanese Laid-Open Patent Publication No. 09-225565.
According to the invention disclosed in Japanese Laid-Open Patent Publication No. 09-225565, the workpiece which is to be provided with the curled portion is a can lid. If the disclosed invention is applied to a wheel rim, then the cuts tend to reduce the rigidity of the curled portion. Consequently, the curled portion fails to perform its primary function to increase the mechanical strength of the rim.